Volumetric flow and mass flow meters are used in many applications and industries. Conventionally, for multiphase flow a collection of separate intrusive flow measurements are required to provide an independent volumetric flow measurement for each phase component of the medium. This invasive technique affects the flow rate, and also restricts the application to situations where the pipeline may be broken into to give access to the medium.
Non-intrusive clamp-on methods of flow metering exist and such meters often use Time Of Flight (TOF) ultrasonic flow measurements that send pulses, typically sent at an angle, from a first transducer through the pipe wall and then fluid flowing in a conduit and measure the time it takes for the sonic waves to reach another one of its transducers. The volumetric flow can then be calculated based on the known internal pipe area. However, TOF technology loses its ability to measure accurately when there are higher levels of gaseous or particle entrainment. Therefore, if the liquid flow has significant entrainment of gas, or solids and slurry, then Doppler technology-based measuring devices are used instead. However, Doppler technology also has limitations and if the process had periods of low entrainment then the Doppler method is less reliable and less accurate and may possibly fail altogether if the entrainment drops significantly. With these existing technologies it is only possible to measure one single phase. The liquid TOF technology measures only one liquid phase, and the Doppler technology only measures the liquid phase during higher entrainment. The gas TOF technology can only measure gas flow when there are low levels of suspended liquid or particle entrainments in the gas flow. Gas with higher levels of suspended liquids or particles is commonly known as wet gas. Existing USM (ultrasonic meter) technology cannot measure wet gas flow, even at relatively low levels of wet gas entrainment. Therefore, it is not possible to use these existing technologies to obtain reliable measurements through phase changes for either liquid or gas with clamp-on nonintrusive technology. Further, it is not possible to measure either the gas part in a liquid flow or the liquid part in a gas flow. Also, without a means of measuring the density of the medium it is not possible to accurately determine the mass of the flow.